Downhole NMR processing

Abstract

An expert system is included in a downhole processor designed to acquire and process NMR data downhole in real time. The downhole processor controls the acquisition of the NMR data based at least in part on instructions transmitted downhole from a surface location and at least in part on evaluation of downhole conditions by the expert system. The downhole conditions include drilling operation conditions (including motion sensors) as well as lithology and fluid content of the formation obtained from other MWD data. The wait time, number of echos, number of repetitions of an echo sequence, interecho time, bandwidth and shape of the tipping and refocusing pulses may be dynamically changed. Data processing is a combination of standard evaluation techniques. Selected data and diagnostics are transmitted uphole. The expert system may be implemented as a two stage neural net. The first stage does the formation evaluation and the second stage controls the NMR pulse sequence.

Description

CROSS REFERENCES TO RELATED APPLICATIONS[0001]This application is a continuation of U.S. patent application Ser. No. 09 / 928,768 filed on Aug. 13, 2001, now U.S. Pat. No. 6,727,696.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention is related to methods for acquiring and processing nuclear magnetic resonance (NMR) measurements for determination of longitudinal and transverse relaxation times T1, and T2. Specifically, the invention deals with use of an expert system downhole for acquiring and evaluating NMR measurements contemporaneous with the drilling of wells and with use of a downlink communication from the surface for modifying the parameters of the downhole acquisition system.[0004]2. Description of the Related Art[0005]Nuclear magnetic resonance is used in the oil industry, among others, and particularly in certain oil well logging tools. NMR instruments may be used for determining, among other things, the fractional volume of pore space and the frac...

AI Technical Summary

Benefits of technology

[0015]The present invention is an apparatus and method for acquiring NMR data of an earth formation using a sensor assembly conveyed on a measurement while drilling device in a borehole in the earth formation. The sensor assembly includes components on a non-rotating sleeve that may be clamped to the formation. With this arrangement, it is possible to continue drilling operations (“making hole”) while making NMR pulse echo measurements at a fixed depth in the borehole and substantially isolated from vibrations caused by drilling. A downhole processor controls the acquisition and processing of the data. The processor controls the acquisition parameters based upon downhole motion sensors and also based upon control signals sent from the surface. Motion sensors such as accelerometers are used to monitor the motion of the sensor assembly and Quality control diagnostics are generated in real time. The processing includes standard processing methods. The basic pulse sequence is a CPMG sequence, although modified CPMG sequences with reduced power consumption may be employed.

Problems solved by technology

Effective porosities are typically summations of partial porosities; however, distortion of partial porosity distributions has been commonly observed for a variety of reasons.

These reasons include poor signal-to-noise ratio (SNR), and poor resolution in the time domain of the NMR data.

Rig time is expensive, so that the general objective in wireline logging is to obtain interpretable data within as short a time as possible.

In contrast, measurements made with a drilling assembly in the wellbore have several problems.

First of all, there is little prior information available about the actual subsurface formations except that inferred from surface seismic data.

This makes it difficult, if not impossible, to base an acquisition scheme on the basis of expected properties of formations.

Secondly, when the drilling assembly is in a borehole, data communication capability is in most cases severely limited.

The data rate with mud pulsing is limited to a few bits per second and communication through the drillstring becomes a serious problem when the drillbit is being operated due to the vibration and noise produced.

This makes it impossible to evaluate acquired data at the surface and to modify the acquisition scheme based on this evaluation.

A third problem arises from the nature of NMR data itself.

This results in a severe degradation of the data.

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